(a) Field of the Invention
The present invention relates to an image transmission optical system for an endoscope and, more particularly, to an image transmission optical system to be used with an inflexible-type endoscope or the like which is arranged to transmit an image by using a plural number of relay lenses.
(b) Description of the Prior Art
An inflexible-type endoscope arranged to transmit an image by using a plural number of relay lenses is composed that an objective 2, image transmission optical systems 3, 3', . . . and eyepiece 4 are arranged in an outer tube 1 in the order from the object side as shown in FIG. 13. The above-mentioned type of endoscope is arranged that an image Q of an object formed by the objective 2 is relayed in turn as Q.sub.1, Q.sub.2, . . . by means of respective image transmission optical systems 3, 3', . . . , and the last image Q.sub.L is observed through the eyepiece 4. The image transmission optical system disclosed in Japanese published examined patent application No. 5993/74 is known as an image transmission optical system to be used with the above-mentioned type of endoscope. Said known image transmission optical system is arranged that two rod-like cemented doublets, each consisting of a positive lens element and negative lens element cemented together, are arranged so that the negative lens elements thereof face each other as shown in FIG. 14.
In case of said known image transmission optical system, spherical aberration, coma and chromatic aberration are corrected favourably. However, curvature of field is not corrected satisfactorily and, moreover, astigmatic difference is caused. Therefore, when the number of image transmission optical systems constituting the endoscope becomes large, curvature of field and astigmatic difference are accumulated and become very large. As a result, it is impossible to bring both of the central portion and marginal portion of the image into focus at the same time.
The image transmission optical system disclosed in Japanese published unexamined patent application No. 4245/77 is known as an image transmission optical system arranged to eliminate the above-mentioned disadvantage. Said known image transmission optical system is composed that two rod-like meniscus lenses 7 and 8, which are arranged so that the concave surfaces thereof face each other, are arranged between two rod-like biconvex lenses 5 and 6 as shown in FIG. 15 and is arranged that curvature of field and astigmatic difference are corrected by means of negative action of an air lens 9 which is formed by those two concave surfaces.
However, when an image is transmitted by using said known image transmission optical system, heights of rays that pass the optical system become the highest at the surfaces 10 and 11 on the inner sides of the biconvex lenses 5 and 6. As a result, as it is evident from FIG. 15, upper rays of offaxial rays are eclipsed at a position (position shown by the arrow mark A in FIG. 15) of long distance from the object point and, consequently, the upper NA becomes small. On the other hand, paraxial rays are eclipsed at the position shown by the arrow mark B. As described in the above, in case of said known image transmission optical system, paraxial rays and offaxial rays are eclipsed at positions which are largely different from each other. Consequently, NA of paraxial rays and NA of offaxial rays become largely different from each other, and the brightness at the central portion of field becomes considerably different from the brightness at the marginal portion of field. The above-mentioned disadvantage becomes very conspicuous especially when the length of the meniscus lenses 7 and 8 is increased in order to prevent the lenses from inclining in the outer tube.